The fermentation of carbohydrates by Clostridium acetobutylicum (hereafter abbreviated C. acetobutylicum) to form butanol and acetone was disclosed by Weizmann in U.S. Pat. No. 1,315,585. For many years, this process was used commercially for the preparation of butanol and acetone, and a certain amount of ethanol was obtained as a by-product.
Eventually, the microbial process was displaced by chemical processes which provide the same products using cheap fossil fuel raw materials. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in the fermentation reaction that uses carbohydrates, which are renewable raw materials.
One reason why the economics of producing acetone and butanol by fermentation has been unattractive has been the cost of the ground corn or high quality molasses feedstock used as the carbohydrate source. This has led to a search for less-expensive feedstocks for the process.
A fermentation utilizing straw as a starting material is described in U.S. Pat. No. 2,417,801. According to this disclosure, a substantial portion of the hemicelluloses in the straw had to be hydrolyzed with an acid before they could be fermented. Hydrolysis of other cellulosic materials was said to release substances deleterious to the fermentation. It was further reported in this patent and in U.S. Pat. No. 2,377,197 that rice bran or wheat bran could be used as the carbohydrate feedstock for the fermentation with C. acetobutylicum. However, the principal carbohydrate that was fermented in these cases was the starch component of the brans, and only a small part of the hemicellulose present was used.
The corn wet-milling industry produces a fibrous residue after the bulk of the germ, starch and gluten is removed from the corn. This residue, known as corn fiber, amounts to about 10% of the dry weight of the corn. It contains about 30% starch, 35% hemicellulose, 15% cellulose and 12% protein with smaller amounts of other components. This fiber would be a low-cost fermentation feedstock if the microorganism causing fermentation could use the carbohydrates present in the fiber.
It has now been found that an asporogenic strain of C. acetobutylicum, Strain ATCC 39,236, is capable of fermenting the starch and a substantial portion of the hemicelluloses present in corn fiber. Furthermore, this strain of C. acetobutylicum can ferment the hemicellulose, xylan, without the need to hydrolyze this polysaccharide before conducting the fermentation.